1. Field of the Invention
The present invention relates to a constant velocity universal joint for any type of transmission and in particular for helicopters with so called “gimballed” rotors, intended to find use in the three blades or in general multiblade main rotors of helicopters, as well as in two-blade main rotors for helicopters also equipped with stabilizer bar aerodynamics.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
As it is known, in helicopters, the control of the direction of motion of translation occurs by means of the inclination of the plane of rotation of the disk, defined by the circular path of the blades, with respect to the driving shaft constrained to the structure of the helicopter.
In helicopters with three or more blades normally rotors called “articulated” are employed, whose blades are bound to the rotor shaft with horizontal hinges, called hinges of flapping, eccentric with respect to the axis of rotation, to allow the motion of flapping, and with vertical hinges (drag hinges), the latter being suitably equipped with friction or provided with viscous devices which act on the plane of the disk to damp oscillations caused by Coriolis accelerations, which affect the blades when the rotor disk is inclined and the flapping motion is present.
Hinges aligned with the longitudinal axis of the blades are also present, at the root of each blade, designed to permit variations induced by the swash plate in the pitch of each blade, by the cyclic and collective pitch commands. In this type of rotors, thanks to the eccentricity of the hinges of flapping hinges and to the centrifugal force of the blades, the inclination of the plane of the disk exerts a favorable moment of control on the driving shaft (generally called “mast”), independent from the lift exerted by the rotor itself and which tends to maintain the same mast normal to the plane of rotation of the blades.
A variant of the articulated rotors is constituted by so-called “hingeless” rotors, in which the hinges are replaced by flexible elements which behave as virtual hinges, also being eccentric. In these types of rotor, such as in the articulated ones, the control of the inclination of the plane of the disk is realized by means of the cyclic oscillating motion of the blades around the pitches hinges, implemented by the swash plate.
There are also two-bladed rotors whose hub is hinged on the top of the driving shaft with a cylindrical hinge perpendicular to the axis of the blades, and to the axis of the mast, functioning as suspension hinge (called “teetering”) which provides flapping motion of the blades and allows the inclination of the rotor disc. In the latter type of rotor, also known as suspended rotor (“seesaw”), it is required that the cylindrical hinge connecting the hub to the shaft lies in a higher position with respect to the attack of the blades, practically on the plane defined from the centers of gravity of the hub and of the blades, inclined upwards due to the lift, in order to cancel the vibrations that would be created for the cyclic displacement of the center of gravity of the rotor relative to the axis of rotation caused by tilting of the rotor disc. The blades, in fact, subject to the centrifugal force due to the rotation, and to lift force, assume an equilibrium position facing upward, so as to set a conical aspect of the disc, with consequent raising of the center of mass of the rotor. The variation of taper angle of the blades, due to different dynamic conditions and to the variability of the weight on board the helicopter, however, makes it impossible to completely eliminate this source of vibrations in all flight conditions and weight of the helicopter.
Furthermore, in this type of rotor, there is another source of vibration which originates from the geometry of the simple gimbal central hinge of flapping that, not being homokinetic, induces torsional oscillating stresses, with a frequency of two per revolution, on the driving shaft; those stresses are translated, thanks to the constraints existing between the transmission and the structure, in bothersome vibrations which propagate to the whole cell of the helicopter and the intensity of which is greater the more the rotor disc is tilted with respect to the shaft.
This type of rotor, moreover, due to the presence of the central hinge of flapping, applies no control moment on the transmission shaft; for this reason, in the flight conditions at low g (n<1 g) or zero-g, it is possible to lose control of the plane of the rotor disc. Even in this type of rotors, the control of the inclination of the disk is realized by means of the oscillating motion of the blades implemented by the swash plate around the pitch hinges.
All types of rotor above described are characterized by considerable stresses on the pitch hinges binding the blades to the hub, countering the centrifugal force; the stresses are caused by the reciprocating motion around the axis of pitch hinges, the motion being due to the action of the cyclic swash-plate action on blades pitch to keep the rotor disc inclined.
To obviate the above mentioned drawbacks various systems have been proposed and disclosed. During the development of laminated elastomeric bearings in the past decades (U.S. Pat. No. 2,481,750 and U.S. Pat. No. 2,900,182), the types of elastomeric bearings that were later applied to the rotors of helicopters (USAAMRDL-TR-75-39B), have been disclosed: axial cylindrical bearings, cylindrical bearings, radial ball bearings and tapered roller bearings. Many of these types of bearings are at the basis of important refinements to the pitch hinges (U.S. Pat. No. 3,111,172 and U.S. Pat. No. 3,652,185) to reduce, but not eliminate, the friction and the consequent efforts on the commands caused by the use of the traditional rolling bearings in the pitch, with also considerable advantages for their durability.
Furthermore, in the field of two-bladed rotors hung with central hinge of flapping, U.S. Pat. No. 4,115,031, by Textron, has disclosed a method to allow the installation of a return spring on the hub (called “Hub spring”) of the rotors suspended around the flapping hinge, in order to obtain a moment of control of the rotor with respect to the driving shaft and the helicopter, to overcome the drawbacks of loss of control in flight to zero-g, by absorbing the consequent the two-per revolution vibrations induced by return spring, by means of the flexibility of the blades in the plane of rotation, tuning the characteristic frequency of oscillation in that plane. However, that solution has not eliminated the torsional oscillation on the mast caused by the geometry of the semi-cardanic transmission of the rotary motion to the rotor.
Moreover, in order to reduce the amplitude of the alternate oscillations on the pitch hinges, caused by the cyclic control, oscillations that occur in conventional rotors described, and in order to eliminate the Coriolis accelerations and then the lag hinges in the rotors with 3 or more blades, some types of rotor so called “gimballed” or also called “floating-hub” have been developed. In these rotors, the central hub is supported on the top of its driving shaft by means of a spherical hinge, or with other kinematically equivalent devices, which allow an inclination of the hub with two degrees of freedom and therefore allow the hub to be always parallel to the plane of rotation of the blades (so called “tip path plane” or TPP), even though they are tilted. The floating-hub rotors have proved themselves to be suitable to be equipped, together with appropriate devices, to create a resilient biasing between hub and shaft, making the behavior of these rotors quite similar to the articulated rotors in flight conditions at low or zero g.
This type of rotor hubs were initially used in aircraft in which the power was transmitted to the blades by means of jets of gas emitted by suitable nozzles located at the end of the blades themselves (system called “Tip Jet” or “Tip driven”). In that type of power transmission the need of the homokinetic transmission of motion from the driving shaft to the hub of the rotor was not requested.
Later on, in some aircrafts (helicopters and convertiplanes), in which the motion of rotation of the blades and the drive torque is transmitted from the driving shaft to the hub, various systems have been developed aimed at eliminating the torsional oscillations on the mast and various solutions have been proposed to achieve constant velocity joints suitable for this type of floating hub rotors. The central spherical hinge, normally made of elastomeric laminates spherical bearings of known type (e.g. U.S. Pat. No. 3,941,433), is equipped with appropriate stiffness so that the rotor disc, when it is tilted, exerts a favorable control moment on the shaft of the rotor, with a behavior similar to articulated rotors fitted with eccentric flapping hinges. A model of this type of rotor has been disclosed by the U.S. Pat. No. 4,729,753 in which the hub of the rotor, suspended spherical by means of two opposed laminated elastomeric bearings on the no transmission shaft, is maintained in rotation by means of suitable elastomeric elements that realize a substantially homokinetic joint.
Many of such solutions have been proposed in view of applications of hybrid rotors (called “tilt rotors”) applicable to convertiplanes. The complexity of these systems has limited their employment opportunities and has prevented the spread in commercial helicopters. In the field of us the rotors with complex mechanical, other authors have disclosed different types of rotary connections (called “link-type rotary coupling”, see U.S. Pat. No. 4,804,352) for three-bladed rotors, in order to obtain a spherical suspension of the rotor and a quasi-homokinetic motion transmission.
Also in other fields of application, some types of constant velocity joints have been proposed, consisting of several assembled elastomeric devices, that allow to transmit the rotary motion with a drive torque between two rotating inclined shafts. An example of these devices has been released by U.S. Pat. No. 4,208,889. In general, the homokinetic cardanic rotors are characterized, so far, by a considerable constructional complexity and their assembly involves the coupling of a very large number of components. The aforementioned elastomeric devices, moreover, do not always guarantee the homokinetic transmission of motion between the shaft and the rotor hub to an acceptable level because of resilient materials which many of the components are made.